How do I combine multiple OpenAPI 3 specification files together?

Most OpenAPI tools can work with multi-file OpenAPI definitions and resolve $refs dynamically.

If you specifically need to get a single resolved file, Swagger Codegen can do this. Below are usage examples for the command-line version of Swagger Codegen. The input file (-i) can be a local file or a URL.

Note: Line breaks are added for readability.

OpenAPI 3.0 example

Use Codegen 3.x to resolve OpenAPI 3.0 files:

java -jar swagger-codegen-cli-3.0.34.jar generate
     -l openapi-yaml
     -i ./path/to/openapi.yaml
     -o ./OUT_DIR
     -DoutputFile=output.yaml

-l openapi-yaml outputs YAML, -l openapi outputs JSON.

-DoutputFile is optional, the default file name is openapi.yaml / openapi.json.

OpenAPI 2.0 example

Use Codegen 2.x to resolve OpenAPI 2.0 files (swagger: '2.0'):

java -jar swagger-codegen-cli-2.4.27.jar generate
     -l swagger-yaml
     -i ./path/to/openapi.yaml
     -o ./OUT_DIR
     -DoutputFile=output.yaml

-l swagger-yaml outputs YAML, -l swagger outputs JSON.

-DoutputFile is optional, the default file name is swagger.yaml / swagger.json.

One way to do this is to use the open-source project speccy.

Open the terminal and install speccy by running (requires Node.js):

npm install speccy -g

Then run:

speccy resolve path/to/spec.yaml -o spec-output.yaml

I wrote a quick tool to do this recently. I call it openapi-merge. There is a library and an associated CLI tool:

• https://www.npmjs.com/package/openapi-merge
• https://www.npmjs.com/package/openapi-merge-cli

In order to use the CLI tool you just write a configuration file and then run npx openapi-merge-cli. The configuration file is fairly simple and would look something like this:

{
  "inputs": [
    {
      "inputFile": "./gateway.swagger.json"
    },
    {
      "inputFile": "./jira.swagger.json",
      "pathModification": {
        "stripStart": "/rest",
        "prepend": "/jira"
      }
    },
    {
      "inputFile": "./confluence.swagger.json",
      "disputePrefix": "Confluence",
      "pathModification": {
        "prepend": "/confluence"
      }
    }
  ], 
  "output": "./output.swagger.json"
}

For more details, see the README on the NPM package.